Field of the Invention
The field of the invention is the making of micro-components belonging to the family of vacuum tubes, of the diode, triode and electroluminescent component type.
In recent years, there has been a renewal of interest in vacuum tubes (especially micro-tubes) for special applications such as very wideband components and components which are hardened to radiation.
This renewal of interest has been stimulated by the application, to the fabrication of tubes, of technological methods used by the electronics industry for micro-machining and the fabrication of semiconductor-based components. For, while it has long been known that manipulating electrons in a vacuum makes it possible to communicate speeds of motion to them which are greater than those obtained in solids, vacuum tube fabrication technologies do not enable any miniaturization compatible with a reduction in the access time to values comparable to those obtained with semiconductor-based components and circuits (notably because of the resistors and capacitors of the control lines). Furthermore, the cathodes of vacuum tubes work by thermionic emission. This requires heating of the source and, therefore, a special environment for the source. The space charge effects in front of thermionic cathodes further require the use of high voltages so as to achieve electrical field values compatible with the removal of a sufficient number of electrons.
The manufacture of systems of tiplet by means of micro-electronic technologies (see for example, C.A. SPINDT et al., Journal of Applied Physics, Vol. 47, December 1976, page 5248) has enabled the making of field emission micro-cathodes with dimensions similar to those of solid state electronic components. These micro-cathodes have been used for the fabrication of electron microguns (U.S. Pat. Nos. 3,743,022, 4,663,599) as well as for the fabrication of matrix networks of electron microguns (U.S. Pat. No. 4,498,592). The recognized advantages associated with the reduction in dimensions are related to the possibility of obtaining very intense electrical fields (&gt;10.sup.7 V/cm) with moderate extraction voltages (some hundreds of volts). In the case of triode type components, if the anode-microcathode distance is small, the space charge effects before the microcathodes can be eliminated for relative low anode/cathode voltage values. Thus, the carriers are accelerated to their final speed more quickly than in solids. This enables reduction in the transit time. The access times (RC of the control lines) are furthermore identical to those obtained in solid state devices, and the speeds of motion of the electrons in a vacuum may, in practice, reach some 108 cm/s as compared with some 10.sup.7 cm/s in the most efficient semiconductors (InSb).
Vacuum microtubes manufactured on the basis of field effect microcathodes thus enable the manufacture of components that are potentially faster than semiconductor-based solid state components.
Components such as this have already been made and, in particular, have been made with technologies used for the fabrication of silicon-based integrated circuits (H.F. GRAY et al., IEDM 1986, 33-1, p. 776).
This is a "planar" type of approach where the source, grid and anode are arranged in one and the same plane which is the upper plane of the substrate.
Another approach has also been used, wherein the anode (10) is a metallic plate facing the cathode (6) and the grid (3) ("stacked" type structure). This structure is shown schematically in FIG. 1. The cathode (6) is formed on a substrate (1), within a cavity (5) hollowed out in a dielectric layer (2).
The drawback of these two structures is that they cannot work except under high vacuum (10.sup.-10 Torr) and that, in particular, various adsorptions are liable to make notable modifications in the emission properties of the tiplets. To avoid this type of phenomenon, means should be provided for in-situ sealing in a glass bulb, or other container, so that the tiplets are not re-exposed to the atmosphere after their manufacture. When a network of tiplets is used, it is also necessary to see to it that the anode/grid distance is strictly maintained so as not to induce any excessive variation in the characteristics of the components. It might also be added that the definition and positioning of an individual anode (of the same dimensional magnitude as the cathode) facing the cathode may rise raise serious technological problems. It is therefore difficult to make each tiplet on an individual basis to make it a transistor type of an elementary component.
The invention enables these different drawbacks to be overcome.